ATLANTA – The Knee Defender, a gadget that blocks airplane seats from reclining, got a global boost after a scuffle between two passengers forced a United Airlines jet to make an unscheduled landing.

The gizmo’s website crashed Tuesday after traffic surged, and sales rose “substantially” for the $21.95 plastic clips that have been on the market since 2003, said the inventor, Ira Goldman.

While a product that interferes with another flier’s comfort may rub some people the wrong way, the issue is airlines’ legroom cutbacks, Goldman said. Carriers are shrinking space between rows — Spirit Airlines’ allotment is about 10 percent less than the industry standard — and using thinner cushions to squeeze more people into coach cabins.

“They don’t have Plan B for the fact that a lot of people, when they sit down in their seat at the gate, their knees already are hitting the seat-back in front of them,” Goldman said in a telephone interview from Washington.

The Knee Defender hit the headlines because of an in-flight squabble on United Flight 1462, which had to touch down Sunday in Chicago en route to Denver from Newark, N.J. One person installed a device that prevented the passenger in front of him from reclining, said Charlie Hobart, a spokesman for United Continental Holdings.

The Associated Press, citing an anonymous law enforcement source, gave a more graphic account: Upset that her seat was locked, one traveler threw water at a man who employed a Knee Defender and refused to remove it at the request of a flight attendant.

Both people were in United’s Economy Plus section, an area in the coach cabin that gives people as much as five more inches of legroom for a fee or elite frequent-flier status at the Chicago-based airline.

Goldman’s gadget is a pair of U-shaped clips that fits over the arms of the seat-back tray table, blocking the passenger in front from leaning back. The Federal Aviation Administration prohibits the use of such devices during taxiing, takeoffs and landings, when tray tables must be stowed. The agency said Tuesday in a statement that airlines can decide whether they want to allow such devices while cruising.

The four largest U.S. carriers — American Airlines, United, Delta Air Lines and Southwest Airlines — all bar the use of the Knee Defender, spokesmen said. JetBlue Airways, the fifth-biggest, discourages the use of the devices while not specifically barring them.

The U.S. industry’s standard is about 30 to 31 inches of space between rows. While Spirit passengers have to make do with only 28 inches, some of their seats aren’t susceptible to the Knee Defender. They don’t recline.

Goldman said he doesn’t know which airlines allow them and which don’t. He likened his invention to a radar detector, which may be legal and tolerated in many states, if frowned upon.

The Wisconsin Assembly planned to finish its work for the year Thursday by approving $350 million to build a new prison and provide all parents a $100 per-child tax rebate, although it's uncertain whether either will pass the Senate.